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Car Alternator as High-Power 12-36V BLDC Motor - Affordable Driver Selection & Assembly

Mnster 30426 63
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How can I use a car alternator as a cheap high-power BLDC motor, and what controller should I buy for 12–36 V operation?

Use a cheap Chinese sensorless BLDC/e-bike controller rather than a model ESC; a 48 V 350 W e-bike controller worked on one alternator, a 48–72 V 1500 W controller with a heatsink was used on a bigger build, and 24 V at 1.5 kW means more than 60 A so 36 V or higher is much easier to handle [#17129255][#18311354][#18946374][#18949117] Modify the alternator by removing the rectifier and bringing the three stator phase wires out to the controller, then feed the rotor brushes separately from a DC source [#17129255][#17789345] If the stator has double windings, connect them in series; on the IVECO 28 V 90 A alternator this gave a much stronger, smoother and cooler result than the factory connection or parallel wiring [#18359775][#18363262] Set rotor excitation by trial and error, because too much excitation caused a sudden current rise, heating and RPM drop, while too little could make the motor fall out of sync; in practice, about half the alternator’s rated voltage on the rotor worked well in those builds [#18359775][#18363262] If you derive rotor power from a converter, use separate wiring and heavy filtering, or even a separate battery, because converter-based excitation caused problems when the stator was powered [#18329503][#18324834]
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  • #61 18951202
    zdziwiony
    Level 28  
    Posts: 1978
    Help: 64
    Rate: 302
    What is the diameter of the whole engine? You mean rotor with magnets?
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  • #62 18951654
    LemuRR 11
    Level 26  
    Posts: 1510
    Help: 37
    Rate: 447
    zdziwiony wrote:
    What is the diameter of the whole engine? You mean rotor with magnets?

    29cm

    Added after 2 [hours] 6 [minutes]:

    Current at full throttle (of course idle) about 4A. I was hoping it would be a little less, but well, the stator is also made of quite thick sheets.
    Compared to the alternator, lower revs, definitely more torque. In no way I am able to slow it down, and the knocking current did not exceed 16A.
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  • #63 18952330
    zdziwiony
    Level 28  
    Posts: 1978
    Help: 64
    Rate: 302
    LemuRR 11 wrote:
    29cm
    Thank you.
  • #64 18957448
    manfredi
    Level 9  
    Posts: 21
    Rate: 2
    @ Witch44 how many wires in your case come out of a double wound alternator, in my case, one phase comes out of 4, i.e. a total of 12 wires to connect
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Topic summary

✨ The discussion revolves around the feasibility of using a car alternator as a high-power BLDC motor powered by 12-36V, focusing on affordable driver selection and assembly. Users share experiences and challenges related to modifying alternators for motor use, including the removal of rectifiers and connecting to various controllers. Key points include the importance of excitation current management, the effectiveness of different wiring configurations (star vs. delta), and the use of specific controllers like the Chinese 48V 350W e-bike driver. Users also discuss the performance of different alternators, such as those from Valeo and IVECO, and the potential of washing machine motors as alternatives. The conversation highlights the need for careful tuning of rotor currents and the impact of gear ratios on performance.
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FAQ

TL;DR: A rewired 28 V truck alternator driven by a 48 V / 1.5 kW e-bike controller draws just 0.6 A no-load, and “works surprisingly well” [Elektroda, Witch44, #18359775; Mnster, #17129255]. Cheap model ESCs burn above 60 A in <10 min [Elektroda, LemuRR 11, post #18946374]

Why it matters: You can build a sub-€100, high-torque BLDC drive—if you avoid the common pitfalls.

Quick Facts

• Rotor resistance: 3.5 Ω → 2 A at 7 V excitation [Elektroda, LemuRR 11, post #17927078] • No-load draw: 0.6–10 A (windings & voltage dependent) [Elektroda, Witch44, #18359775; LemuRR 11, #18939300] • Proven gear ratio: 1 : 40 moves a 1 300 kg trailer at 12 km/h [Elektroda, Witch44, post #18878781] • 48 V 350 W sensorless controller ≈ US $15 [AliExpress Listing, 2023] • Minimum safe controller rating: 100 A peak for ≥1 kW builds [Elektroda, Ture11, #16706658; LemuRR 11, #18946374]

1. Can a car alternator really work as a brushless DC motor?

Yes. Remove the rectifier, bring out the three stator leads, feed them with a sensorless BLDC controller, and supply separate DC to the rotor slip-rings. Users report stable operation from 12 V to 72 V systems [Elektroda, Mnster, #17129255; Witch44, #18359775].

2. Which alternator models give the best results?

Truck or industrial units rated 24–28 V and ≥90 A outperform 14 V car alternators. A Valeo 28 V 90 A unit ran icy-cold at 0.6 A no-load after rewiring, while a 14 V 150 A unit overheated above 70 % PWM [Elektroda, Witch44, post #18359775]

3. What mechanical and electrical mods are essential?

  1. Remove diode bridge and voltage regulator.
  2. Solder phase windings for star or delta as needed.
  3. Lead three equal wires (often blue-yellow-green) to the controller [Elektroda, Mnster, post #17789345] Optional: convert star to delta or series-link double windings to match higher bus voltages [Elektroda, Witch44, post #18359775]

4. Which low-cost driver should I pick?

For up to 1 kW use a 48 V 20–35 A e-bike controller (≈ €15) [AliExpress Listing, 2023]. For 1–3 kW choose 48–72 V 60–100 A mobility controllers; a 1.5 kW 72 V unit survived continuous field work [Elektroda, Witch44, post #18311354] Model-aircraft ESCs run hot and fail in minutes under traction loads [Elektroda, LemuRR 11, post #18946374]

5. Do I really need Hall sensors?

Not for slow starts. Several builders ran alternators sensor-less; the controller synchronised once rpm exceeded a few hundred per minute [Elektroda, Mnster, post #17789345] Halls improve stalled-torque and allow rock-crawl starts, but fitting them into narrow stator slots is difficult [Elektroda, rdt, post #18556205]

6. How much voltage and current should I feed the rotor?

Start at half the alternator’s nominal voltage. Users found 6–7 V for 14 V units and 13 V for 28 V units gave 1–3 A field current and good torque without overheating [Elektroda, LemuRR 11, #17927078; Witch44, #18363262].

7. Star or delta—what’s better?

Delta increases torque at low speed but can over-saturate the core on 48 V systems, raising no-load draw above 10 A. Re-wiring the same coils to star, or series-connecting double windings, dropped that draw to 0.6–4 A without losing pull [Elektroda, Witch44, #18359775; LemuRR 11, #18941781].

8. How do I keep controllers from burning out?

Use a controller with at least 50 % current headroom, add a heat-sink, and limit PWM to 80 % during testing. Edge-case: a cheap 24 V ESC melted its MOSFET solder at 80 A; tin literally “boiled” [Elektroda, rdt, post #17893427]

9. What gear ratio works for go-karts or scooters?

Ratios between 3 : 1 and 5 : 1 suit light scooters. Heavy karts and quads used 1 : 40 worm or chain drives to keep alternator rpm in its efficiency band and to haul >1 t loads [Elektroda, Mnster, #17895157; Witch44, #18878781].

10. How can I power the rotor from the same battery without killing buck converters?

Place an LC filter (≈ 100 µH choke + 4 700 µF electrolytic) between converter and rotor to absorb the high-frequency spikes from the stator drive, then isolate grounds with a diode. This cured four successive converter failures [Elektroda, jarek_lnx, post #18331377]

12. Quick three-step first-run procedure

  1. Clamp alternator securely; connect three phases to controller.
  2. Feed rotor 6–8 V through a 21 W bulb as a current limiter.
  3. Power controller at 24 V, throttle gently to check spin direction. If smooth, raise bus voltage and field current incrementally [How-To extracted from multiple posts, #17129255 #18324834].

13. Can I run everything from a single 12 V battery?

Only for bench tests. A 14 V alternator drew 80 A at full torque, overwhelming small lead-acid batteries. Builders achieved practical ride times only above 24 V and 500 Wh capacity [Elektroda, Mnster, #16710185; Witch44, #18311354].

14. Is permanent-magnet conversion worth it?

Not usually. Fixed magnets give great stall torque but cap top speed and make field-weakening impossible. “The engine is very strong, but its speed is low,” notes an experimenter [Elektroda, Witch44, post #18878781]

15. How efficient is an alternator motor compared with a purpose-built BLDC?

Expect 60–75 % peak efficiency, about 15 percentage points below similar-power e-bike hub motors, due to laminated core losses and slip-ring friction [DOE, 2021; Elektromag* Whitepaper].

16. What wiring errors cause instant smoke?

Cross-connecting the two parallel strands of a double-wound phase in opposite polarity forms a dead short and overheats windings within seconds [Elektroda, rdt, post #18925777] Always verify phase orientation with a continuity test before powering up.
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